Air conditioning systems can be used to cool a plurality of separate spaces in the same building or a plurality of individual buildings in a defined geographic area or district as, for example, a university campus, a hospital, an office park, a governmental building complex, a shopping mall complex, or a theme or entertainment park.
Existing district cooling systems use a refrigeration apparatus which produces cold water, rather than an aqueous ice slurry, in a large central refrigeration system. The cold water, typically at 40.degree. F., is fed through a piping network to the air handlers in the buildings where the building air is cooled. The resulting warm water at 55.degree. F. is returned to the central refrigeration system to be cooled and then reused for cooling purposes. A major cost component for such district cooling system is the piping network which transports the cold supply water and the warm return water.
In recent years apparatus and methods have been developed for the production of aqueous ice slurries, as see the Knodel et al U.S. Pat. No. 4,596,120 and Schoerner et al U.S. Pat. No. 4,452,302. Aqueous ice slurries have many uses such as in food processing and storage, industrial processes and in air conditioning.
Mohlman U.S. Pat. No. 3,247,678 discloses the use of an ice-brine slurry for air conditioning. An ice-brine slurry from a single refrigeration station is used to supply a plurality of paying users with refrigeration. Air to be cooled is passed in indirect heat exchange relationship with the ice-brine slurry whereby the latent heat for melting of the ice is used to cool the air. The warm water is returned directly to the refrigeration plant in which the ice slurry is produced. The ice slurry is sent to a head tank for storage rather than being sent directly to the paying users. This system will typically require mechanical agitation to keep the stored slurry in a fluid state.
Kuehner U.S. Pat. No. 3,869,870 discloses production of a slurry of ice crystals in a water immiscible carrier, such as toluene, which is circulated to one or a plurality of heat exchangers where it flows through the coils for absorbing heat from a load by means of the melting ice. The two liquids are then forwarded to a receiver where they separate into two phases. The liquid water from the melted ice and the carrier fluid are returned separately to the ice forming unit prior to recirculating the slurry through the system. The process does not include thermal energy storage, such as ice stored in a tank, for any purpose.
Newton U.S. Pat. No. 3,906,742 discloses direct mixing of water with a water-immiscible liquid refrigerant which does not form hydrates with water. An ice slurry in liquid refrigerant is circulated to one or a plurality of individual air handling units which may be connected in a closed loop single pipe system. The ice-refrigerant slurry is stored at high concentration ready for recirculation when needed and typically requiring agitation to keep the slurry in a fluid state. The reference does not disclose formation of an aqueous ice slurry with water as the carrier in a refrigeration plant and direct feeding of the resulting slurry with water as the carrier to a heat exchanger means. The reference states that liquid refrigerant, not water, is the principal carrier of the ice crystals to the heat exchanger. Also, the reference does not disclose the use of thermal storage of ice in a storage tank for cooling the returning fluid from the heat exchanger means.
Pronger et al U.S. Pat. No. 4,584,843 discloses that ice can be stored in a tank and be used to cool water for air conditioning purposes. Cold water can be removed from the tank and fed through a heat exchanger cooling coil in a building or the like for cooling purposes. Warm water removed from the heat exchanger can be returned to the storage tank to be cooled by direct contact with the ice in the tank. The system of this reference provides the load management benefits of thermal storage but not ice slurry circulation benefits.
The latent heat of ice, combined with the additional sensible heat available due to the lower temperature, greatly increase the energy absorbing capacity per pound of aqueous ice slurry circulated compared to water. In new systems, the size of the distribution piping to meet a given cooling load can be substantially lowered. This is because the increase in the energy capacity of the ice slurry can be 3.5 times greater than that of chilled water. This can be translated into a decrease in flow of over 70% to meet the same cooling load.
The above-described potential advantages in using an aqueous ice slurry are not readily achievable with many of the air handlers now in use and commercially available because the air handler heat exchanger coil is often incapable of handling an ice slurry, for which they were not designed. Areas of slow flow, or recirculating flow, allow the ice to accumulate and block the coil. Also, a lower flow rate for an ice slurry and a decrease in pumping power for an ice slurry can lead to a reduction in the heat transfer coefficient within the coil so that to maintain capacity either a large temperature difference or more coil surface area is needed. Finally, existing air handlers were not designed to handle the very low temperatures associated with ice slurries. A conventional system typically operates with a 40.degree. F. entering water temperature and a 55.degree. F. exiting water temperature. If the water is too cold, moisture in the air may condense on the coil surface causing the air to be dry and equipment to corrode.
From the above discussion it is clear that a need exists for apparatus and methods which will eliminate some or many of the above problems yet maintain many of the advantages inherent in the use of aqueous ice slurries for various cooling purposes, including air conditioning, especially for district cooling. There is also a need for central thermal energy storage, such as ice in a tank, which can be efficiently used in combination with a refrigeration system which produces an aqueous ice slurry which is used as the primary source for cooling purposes.